So I'm on the road and need to do some editing. I'm finding my laptop is getting very glitchy in playback, so I'm suspecting my storage isn't optimised. Can anyone give some advice on how best to set up my system?

I have a 256GB internal SSD running on a sata3 connection running my OS.

Test all your storage devices with CrystalDiskMark and let us see the results,

EDIT

A few USB devices are good, some are terrible.

Harm's (RIP) basic hard disk drives suggestions are now obsolete in this day and age for editing. Unfortunately, that is my website but it is to complicated and time consuming to just be edited. This is the day of SSD's and not just the old SATA III SSD devices. My i7-5960X almost two-year old deskside computer does not have any hard disk drives in the normal editing mode. It has removable hard disk drives for occasional access to old media and projects and aslo for backup and archiiving

Yeah it was very sad to hear about Harm, even though we never interacted, his comments suggested he was a nice guy who knew his stuff.

Here are my results: (sorry for the delay, the internet in my Mexican hotel has been down)

On the left (S:) is the 256GB USB 3 SSD, the middle is my 4TB USB 3 HDD, and the right is my 256GB internal SSD

The amount of footage for this project should be sub 200GB. I will shortly be clearing space on my c: drive too btw.

***edit*** I have now discovered that when I do a large file transfer from the USB3 HDD to the USB3 SSD, it maxs out at 70MB/s. I'm wondering if this is because the USB3 bus has to share speeds when two USB3 drives are used?

That could be a possibility but also your USB3 HDD just has either a lousy USB3 controller, or most likely as you said it is "nearly full" and you are in a very slow region, for editing purposes we nover recommend much over 50% fullness, archiving and backup it does not make much difference I have actually seen USB3 flash drives that are that slow.

It appears that you might have a Samsung portable T1 or T3. I have great success putting all my project and media on my T1 and using the Internal SSD for OS/Applcations and leaving Premiere's default Media Cache and Media Cache Files on it also.

Have you tuned your OS? One big things is to turn off Indexing on you editing drives.

Also delete all files from the Media Cache and Media Cache Files folder regularly. They rebuild when you reopen a project but there is no automatic cleanup.

I have not been able to test the 100's of clips, but you might try the simple approach and if your project initially loads slowly (especially after cleaning your cache) that would be a sign that it should be on a fast separate SSD